Method and apparatus employing external light source for endpoint detection

ABSTRACT

A method and apparatus for endpoint detection for the stripping of a particular material, such as photo-resist material, from a substrate surface. A beam of light is projected onto the substrate surface and the fluoresced and/or reflected light intensity at a particular wavelength band is measured by a light detector. The light intensity is converted to a numerical value and transmitted electronically to a control mechanism which determines the proper disposition of the substrate. The control mechanism controls the cessation of the stripping process and may control a substrate-handling device which sequentially transfers substrates to and from a stripping chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the manufacture of semiconductordevices prepared by a method including photolithography. Moreparticularly, this invention pertains to a method for inspectingsemiconductor substrates to determine the completion of stripping("endpoint") during a plasma stripping process to remove a photo-resistmaterial from a semiconductor substrate surface after photolithography.

2. State of the Art

Semiconductor chips are produced in a multi-step process by which aplurality of identical electronic circuits is typically formed on asemiconductor substrate, such as a silicon wafer. The semiconductorsubstrate is then subdivided (diced) into individual chips which arefurther processed into packaged semiconductor devices or otherwisesecured in higher-level packaging for ultimate use.

The electronic circuits are generally patterned into a semiconductorsubstrate by a series of steps including photolithography. To elaborate,a photo-resist material is coated onto the semiconductor substratesurface. As disclosed in commonly owned U.S. Pat. No. 5,350,236 issuedSep. 27, 1994, hereby incorporated herein by reference, the temperatureof a semiconductor substrate during the application of a material can bemonitored by measuring light reflected from a surface of thesemiconductor substrate, such that the material and semiconductorsubstrate are not overheated.

After the photo-resist material has been coated on the semiconductorsubstrate surface, it is selectively exposed to a radiation source, suchas by the passage of radiation (i.e., light, e-beam, or X-rays) througha mask having a desired aperture pattern defined therein. If a positivephoto-resist material is used, the exposure to the radiation sourceconverts the positive photo-resist material to a more soluble statewhich allows the exposed positive photo-resist to be removed with asolvent, thereby leaving a pattern substantially identical to the mask.If a negative photo-resist material is used, the exposure to theradiation source converts the negative photo-resist material to a lesssoluble state which allows the unexposed positive photo-resist to beremoved with a solvent, thereby leaving a pattern substantiallyidentical to the openings in the mask. Whether a positive or a negativephoto-resist material is used, the photolithographic process results ina photo-resist pattern which will become the electronic circuit patternon a semiconductor substrate.

Following the removal of the portions of the photo-resist material inthe development process, the semiconductor substrate is subjected tofurther processing steps which may include doping, etching, and/ordeposition of conductive materials in unprotected areas, i.e., areasdevoid of photo-resist material. After one or more of these processingsteps, the semiconductor substrate is subjected to a stripping step toremove the photo-resist material remaining on the semiconductorsubstrate.

The stripping of photo-resist material is commonly achieved using plasmaetching. In plasma etching, a glow discharge is used to produce at leastone reactive species, such as atoms, radicals, and/or ions, fromrelatively inert gas molecules. Basically, a plasma etching processcomprises 1) is generated at least one reactive species in a plasma froma bulk gas, 2) the reactive species diffuses to a surface of a materialbeing etched, 3) the reactive species is absorbed on the surface of thematerial being etched, 4) a chemical reaction occurs which results inthe formation of at least one volatile by-product, 5) the by-product isdesorbed from the surface of the material being etched, and 6) thedesorbed by-product diffuses into the bulk gas. The materials used asphoto-resist are generally organic polymers, such asphenol-formaldehyde, polyisoprene, poly-methyl methacrylate, poly-methylisopropenyl ketone, poly-butene-1-sulfone, poly-trifluoroethylchloroacrylate, and the like. Such photo-resist materials are generallyetched in plasmas containing pure oxygen to produce species that attackthe organic materials to form CO, CO₂, and H₂ O as volatile by-products.

After the removal of the photo-resist material, a subsequent processingstep may include heating the semiconductor substrate in a diffusionfurnace or applying a layer of material with a chemical vapor depositionsystem. Occasionally, a semiconductor substrate is inadvertently passedto a thermal furnace or vapor deposition system with incomplete removalof the photo-resist material. The resulting damage to the processingequipment may be severe. For example, furnace diffusion tubes areirreparably damaged by vaporized hydrocarbons and carbon from thephoto-resist material and, thus, the furnace diffusion tubes must bereplaced. The replacement equipment and/or the downtime to repair theprocessing equipment is usually very costly.

Furthermore, the photo-resist carrying semiconductor substrate and oneor more subsequent semiconductor substrates entering the processingequipment prior to shutdown of the equipment are usually alsocontaminated and must be discarded. At a late stage of manufacture, asemiconductor substrate may have a value between about $10,000 and$20,000. Thus, even an occasional loss is significant.

Therefore, it is very important that completion ("endpoint") of thephoto-resist stripping be accurately detected. A common endpointdetection method with plasma etching is disclosed in U.S. Pat. No.4,377,436 issued Mar. 22, 1983 to Donnelly et al. wherein endpointdetection during plasma-assisted etching is signaled by cessation oronset of spatially confined luminescence resulting from an etch reactionproduct. The light source for the luminescence comes from the plasmageneration. However, as the use of microwave plasma etching hasdeveloped, the generation of the plasma has been removed from theetching chamber. The removal of the plasma generation from the etchingchamber prevents excess heat buildup in the etching chamber caused bythe plasma generation and allows for different frequencies andwavelengths to be used to create free radials (i.e., the reactivespecies).

The reactive species is formed remotely in a microwave reaction chamberand transported to the etching chamber, such as shown in U.S. Pat. No.5,489,362 issued Feb. 6, 1996 to Steinhardt et al. No plasma is presentin the stripping chamber with such a microwave plasma system. Therefore,there is no light source present in the chamber that can be used fordetection of the endpoint removal of the photo-resist material.

Therefore, it would be advantageous to develop an apparatus and methodof luminescent endpoint detection for the stripping of materials in amicrowave plasma etching system employing a plasma chamber separate fromits etching chamber.

SUMMARY OF THE INVENTION

The present invention is an automated method and apparatus fordetermining the endpoint of the removal of a photo-resist material onthe surface of a semiconductor substrate by the detection offluorescence, reflection, or absorption of light by the photo-resistmaterial. Hereinafter, the term "emanated light" is defined as the lightresulting from a light striking the photo-resist material or othermaterial including fluoresced light, reflected light, or absorbed light.

As mentioned above, photo-resist materials are generally organicpolymers, such as phenol-formaldehyde, polyisoprene, poly-methylmethacrylate, poly-methyl isopropenyl ketone, poly-butene-1-sulfone,poly-trifluoroethyl chloroacrylate, and the like. Organic substances cangenerally fluoresce (luminescence that is caused by the absorption ofradiation at one wavelength followed by nearly immediate re-radiation ata different wavelength) or will absorb or reflect light. Fluorescence ofthe photo-resist material at a particular wavelength, orreflection/absorption by the photo-resist material of light at a givenwavelength, may be detected and measured, provided the material differsfrom the underlying semiconductor substrate in fluorescence orreflection/absorption at a selected wavelength or wavelengths. Forexample, a positive photo-resist generally fluoresces red or red-orangeand a negative photo-resist generally fluoresces yellow.

In a particular application of the invention, the presence ofphoto-resist material on a semiconductor substrate surface may berapidly and automatically determined, recorded, and used to determinewhen the photo-resist material has been removed from the semiconductorsubstrate surface. In a preferred application of the present invention,a semiconductor substrate is introduced into a stripping chamber whichreceives at least one reactive species, usually generated from oxygen,from a microwave plasma generator. The stripping chamber includes afirst optical port and a second optical port positioned in a wall of thestripping chamber. A beam of light from a lamp passes through the firstport, strikes the photo-resist material on the semiconductor substrateand is reflected as an emanated beam at an angle through the secondoptical port. Preferably, the photo-resist material differs from thesemiconductor substrate in fluorescence, absorption, and/or reflectionproperties at some wavelengths of incident light.

The intensity of the emanated light will decrease when the photo-resistis stripped away. When the intensity has decreased to a level indicatingthat the photo-resist has been completely stripped away, the strippingprocess can be terminated. This detection method also allows the systemto generate an error signal if the level indicating that thephoto-resist has been stripped is not reached within a certain amount oftime. Such an error signal would indicate that a semiconductor substratewas stripping poorly (i.e., too slowly) or the stripping equipment wasnot functioning properly. This error signal allows for the culling ofthe offending semiconductor substrate for rework or allows for thestripping equipment to be shut down for repair, which prevents thespread of photo-resist material contamination throughout other processsteps. Furthermore, the throughput of the stripping equipment can beincreased because empirically established finite strip times used inconjunction with endpoint detection of the photo-resist removal preventsthe need for exaggerated strip times to ensure complete stripping.

In this invention, the semiconductor substrate is irradiated with light,which light may be monochromatic, multichromatic, or white. In onevariation, the intensity of generated fluorescence particular to thephoto-resist material at a given wavelength is measured. In anothervariation, the intensity is measured at a wavelength which is largely oressentially fully absorbed by the photo-resist material. In a furthervariation, the intensity of reflected light is measured at a particularwavelength highly reflected by the photo-resist material but absorbed bythe substrate.

The intensity of the emanated light is measured by a sensing apparatusand the result inputted to a logic circuit, e.g., a programmablecomputer. The result may be recorded and used for a decision making stepor to activate a culling device.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention can be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIG. 1 is a diagrammatic view of a photo-resist material strippingapparatus of the present invention;

FIG. 2 is a side view of an alternate photo-resist material strippingapparatus of the present invention; and

FIG. 3 is a side view of an alternate light detection configuration ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a stripping apparatus 100 of the present invention.It should be understood that the apparatus 100 of FIG. 1 is not meant tobe an actual view of any particular stripping device, but is merely anidealized representation which is employed to more clearly and fullydepict the process of the invention than would otherwise be possible.

The stripping apparatus 100 comprises a stripping chamber 102 having oneor more entryways or portals (not shown) for the introduction andremoval of semiconductor substrates, such as semiconductor substrate104, into and from the stripping chamber 102. The semiconductorsubstrate 104 may be a semiconductor material comprising a slice ofcrystalline silicon (silicon wafer) or may include varioussemiconductive material or material layers, including without limitationsilicon wafers, silicon-on-insulative (SOI) structure,silicon-on-sapphire (SOS) structure, gallium arsenide, or germanium.

The stripping apparatus 100 also includes a microwave plasma generator106 which generates reactive species in a plasma from an oxygencontaining gas 108 fed to the plasma generator 106. The reactive speciestravel down waveguide 110 into the stripping chamber 102.

A photo-resist material detection apparatus is integrated with thestripping chamber 102 for in situ automated determination of theprogress in stripping of a photo-resist material 114 from thesemiconductor substrate 104. Preferably, the photo-resist material 114differs from the semiconductor substrate 104 in flourescing, absorption,and/or reflection properties at some wavelengths of incident light. Thesemiconductor substrate 104 is shown on a movable stage 118 within thestripping chamber 102 to provide the desired positioning of thesemiconductor substrate 104 with respect to a primary high energy beam134. The movable stage 118 may be movable by one or more stepper motors120 (shown in shadow lines) or other motive means controlled byelectronic signals 122 from a control mechanism 124, such as aprogrammed general purpose computer, i.e., a personal computer drivingappropriate switches.

The photo-resist material detection apparatus includes two opticalports, a first optical port 126 and a second optical port 128, which arepositioned in an upper wall 132 of the stripping chamber 102. Theprimary high energy beam 134 of light from a high energy lamp 136 passesthrough the first port 126, strikes the photo-resist material 114 of thesemiconductor substrate 104 at an angle of incidence α and is reflectedas an emanated beam 138 at an angle of departure β (substantially equalto angle of incidence α) through the second optical port 128. Althoughthe beam 134 may irradiate the entire surface of the semiconductorsubstrate 104 simultaneously, the beam 134 is preferably a sheet beamhaving a width (perpendicular to the plane of the drawing sheet)approximately the width of the semiconductor substrate 104. Thesemiconductor substrate 104 can be passed under the sheet beam usingmovable stage 118, enabling the inspection of the entire surface of thesemiconductor substrate 104. Furthermore, as illustrated in FIG. 2, thesemiconductor substrate 104 can be positioned on a rotating platform180, wherein a sheet beam 182 is directed to a center point 184 of thephoto-resist material 114 on the semiconductor substrate 104 and extendsacross the width (perpendicular to the plane of the drawing sheet) ofthe semiconductor substrate 104 resulting in emanated beam 190. Therotatable platform 180 is rotated about axis 186 such that the entiresurface 188 of the photo-resist material 114 is contacted by the sheetbeam 182. This allows for different perspectives of the photo-resistmaterial surface 188 which will detect photo-resist material 114 thatmay be in a "shadow" due to the topography of the semiconductorsubstrate 104, if only one particular perspective is taken.

Fluoresced and/or reflected light produced by existing photo-resistmaterial 114 in response to the beam 134 is also present in the emanatedbeam 138. The emanated beam 138 may be passed through an optical bandpass filter or suppression filter 140 to absorb non-fluoresced light orundesired reflected light and produce a filtered light beamsubstantially free of such undesired wavelengths. For example, theemanated beam 138 may be passed through the optical band pass filter 140to produce a light beam having a narrow wavelength band of, for example,700 nm±80 nm. Such a wavelength is a characteristic fluorescing emissionof commonly used positive photo-resist materials, as listed above.

The emanated beam 138 is transmitted into a photo-multiplier tube 142for the ultimate generation of an electronic signal 156 indicative ofthe light intensity at the filtered light wavelength. The electronicsignal 156 may be generated by a light intensity sensor 150, such as asilicon diode sensor, which generates an analog intensity signal 152.The intensity signal 152 is sent to a power meter 154 including ananalog-to-digital converter, which converts the intensity signal 152into an electronic binary numerical value comprising the electronicsignal 156. The electronic signal 156 is preferably processed by asoftware program in the control mechanism 124 (preferably a programmedcomputer). It is, of course, understood that analog to digitalconversion is not a necessary limitation. The control mechanism 124 canbe configured to receive an analog signal directly.

The control mechanism 124 determines whether stripping endpoint hasoccurred and sends a cessation signal 160 to the microwave plasmagenerator 106 if endpoint is detected, or if the endpoint is notdetected within a predetermined time frame. The control mechanism 124also provides transfer instructions 162 to a wafer transfer device 148based on electronic signal 156. The transfer instructions 162 aregenerated for either the detection of stripping endpoint or for therejection of the semiconductor substrate 104. The transfer instructions162 will trigger the placement and retrieval of the semiconductorsubstrate 104 into the stripping chamber 102 and from the strippingchamber 102 after the test to another location for further processing.The electronic signals 122 for stage control are also sent by thecontrol mechanism 124 for controlling motion of the movable stage 118.

As illustrated in FIG. 1, the beam 134 is shown striking thephoto-resist material 114 on the semiconductor substrate 104 at theangle of incidence α of about 45 degrees and the emanated beam 138 isshown reflected at the angle of departure β of about 45 degrees. Theincident angle α for the beam 134 and the departure angle β for theemanated beam 138 are preferably between 0 and 45 degrees. However, byusing a dichromatic mirror 172 (a mirror which reflects wavelengths ofless than a given value, and passes wavelengths greater than the givenvalue) as shown in FIG. 3, the beam 134 and the emanated beam 138 mayboth pass through the same port, and incident angle α and the departureangle β are both 90 degrees (i.e., perpendicular to the semiconductorsubstrate 104). The emanated beam 138 is shown offset from the beam 134for the sake of clarity.

The high energy lamp 136 is preferably a mercury or xenon lamp whichproduces high intensity, fluorescence-inducing illumination. The lightoutput from the high energy lamp 136 may be filtered by a band pass orexcitation filter 144 for removing wavelengths from the primary highenergy beam 134 which do not stimulate fluorescence, reflect, or absorbin the semiconductor substrate 104.

As indicated, the method depends upon a difference in fluorescence orlight absorption/reflectance between the material to be detected, e.g.,the photo-resist and the underlying substrate. A wavelength of incidentillumination is typically chosen which maximizes the difference influorescence, absorption, or reflectance. It is preferred to usefluorescence as the measured output, but light absorbance may be usedwhen the material to be detected strongly absorbs a particularwavelength of radiation while the substrate strongly reflects the same.

It should be understood that references herein to light of a particular"wavelength" encompass wavelength bands that are "about" a particularwavelength. In other words, the term "a particular wavelength" mayinclude wavelengths both slightly longer and shorter than the"particular wavelength".

Having thus described in detail preferred embodiments of the presentinvention, it is to be understood that the invention defined by theappended claims is not to be limited by particular details set forth inthe above description, as many apparent variations thereof are possiblewithout departing from the spirit or scope thereof.

What is claimed is:
 1. A method for stripping endpoint detection of aphoto-resist material on a substrate surface, comprising:positioning asubstrate within an etching chamber to receive illumination from a beamof generated light from a light source; etching said photo-resistmaterial on said substrate surface; irradiating said photo-resistmaterial and any exposed portions of said substrate surface with saidbeam of generated light; collecting emanated light from the irradiatedphoto-resist material and said substrate surface portions; filteringsaid emanated light to at least one wavelength indicative of saidphoto-resist material; and generating an electronic signal indicative ofan intensity of said filtered light.
 2. The method of claim 1, furthercomprising transmitting said electronic signal to a control mechanismfor processing.
 3. The method of claim 2, wherein said control mechanismgenerates an instruction to discontinue etching said photo-resistmaterial on said substrate surface.
 4. The method of claim 2, whereinsaid control mechanism generates an instruction for transmission to anautomated substrate handling apparatus to control disposition of saidsubstrate based on said indicated light intensity.
 5. The method ofclaim 4, further comprising moving said substrate by said automatedsubstrate handling apparatus to a location designated to receivesubstrates having the indicated light intensity.
 6. The method of claim5, further comprising the steps of sequentially positioning additionalsubstrates to be irradiated by said beam of generated light formeasurement of light reflected and flouresced from said substrates. 7.The method of claim 2, further comprising transmitting said electronicsignal to a programmable computer for processing.
 8. The method of claim1, further comprising determining the presence of said material bydetecting the presence of a selected wavelength of fluoresced lightcharacteristic of said photo-resist material.
 9. The method of claim 8,further comprising filtering said beam of generated light with a filterprior to irradiating said material to remove non-fluorescence producinglight wavelengths from said beam of generated light.
 10. The method ofclaim 1, further comprising determining the presence of saidphoto-resist material by detecting the absence of a given wavelength oflight characteristically absorbed by said photo-resist material andcharacteristically reflected by said substrate.
 11. The method of claim10, further comprising filtering said beam of generated light through afilter prior to irradiating said substrate surface to limit lighttransmission to wavelengths substantially absorbed by said photo-resistmaterial and substantially reflected by said substrate.
 12. The methodof claim 1, further comprising determining the presence of saidphoto-resist material by detecting the presence of a given wavelength oflight characteristically reflected by said photo-resist material andcharacteristically absorbed by said substrate.
 13. The method of claim12, further comprising filtering said beam of generated light through afilter prior to irradiating said substrate surface to limit lighttransmission to wavelengths substantially reflected by said photo-resistmaterial and substantially absorbed by said substrate.
 14. The method ofclaim 1, further comprising determining the presence of saidphoto-resist material by detecting the presence of at least onewavelength band indicative of the presence of said photo-resistmaterial.
 15. The method of claim 1, wherein generating said electronicsignal includes passage of said filtered emanated light through aphoto-multiplier tube to generate said signal indicative of said lightintensity.
 16. The method of claim 1, wherein said substrate comprises asemiconductor substrate.
 17. The method of claim 2, further comprisingmoving said substrate under said beam.
 18. The method of claim 17,further comprising positioning said substrate on a movable stage formoving said substrate for detection testing of an entire surface of saidsubstrate.
 19. The method of claim 18, wherein movement of said movablestage is controlled by said control mechanism.
 20. The method of claim1, further comprising positioning said substrate on a rotating platformfor rotating said substrate for detection testing of an entire surfaceof said substrate.
 21. The method of claim 1, wherein said beam is asheet beam.
 22. The method of claim 21, wherein said sheet beam has awidth at least as wide as a width of said substrate.
 23. An apparatusfor determining an endpoint for stripping of a material from a surfaceof a substrate, comprising:a stripping chamber for receiving saidsubstrate; a microwave generator for generating at least one reactivespecies for delivery to said stripping chamber for etching saidmaterial; a primary high energy light source; first optical apparatusfor forming a beam of high energy light and directing the high energylight to said material on said substrate; second optical apparatus forcollecting emanated light from said material as a secondary light beamand directing said secondary light beam through a band pass filter; anda light intensity sensing apparatus for receiving said filteredsecondary light beam, measuring an intensity thereof, and generating anelectric signal representative of said measured light intensity.
 24. Theapparatus of claim 23, further comprising a control mechanism forprocessing said electronic signal.
 25. The apparatus of claim 24,further comprising an automated substrate handling apparatus for movingsaid substrate to and from said stripping chamber.
 26. The apparatus ofclaim 25, further comprising a plurality of sites for selective movementof said substrate thereto from said stripping chamber by said automatedsubstrate handling apparatus dependent upon the light intensitymeasurement from said substrate.
 27. The apparatus of claim 23, furtherincluding a platform which is rotatable for measuring an entire surfaceof said substrate.
 28. The apparatus of claim 23, further including astage which is moveable for positioning said substrate for measuring anentire surface of said substrate.
 29. The apparatus of claim 24, whereinsaid control mechanism comprises a computer programmed to receive andrecord said light measurement, instruct said movable stage to move saidsubstrate, and instruct a robot to move said substrate to and from saidmovable stage.
 30. The apparatus of claim 23, wherein said first opticalapparatus comprises a lens and a primary band pass filter forrestricting said beam of high energy light to a predetermined wavelengthband.
 31. The apparatus of claim 30, wherein said primary band passfilter comprises a filter for passing radiation which inducesfluorescence in said material.
 32. The apparatus of claim 30, whereinsaid primary band pass filter is configured to pass light wavelengthswhich are substantially absorbed by said material and substantiallyreflected by said substrate.
 33. The apparatus of claim 30, wherein saidprimary band pass filter is configured to pass light wavelengths whichare substantially reflected by said material and substantially absorbedby said substrate.
 34. The apparatus of claim 30, further comprising adichromatic mirror for reflecting said beam of high energy light througha lens to said surface of said substrate and for passing fluoresced andreflected light in a reverse direction.
 35. The apparatus of claim 23,wherein said high energy light source comprises a mercury lamp.
 36. Theapparatus of claim 23, wherein said high energy light source comprises axenon lamp.
 37. The apparatus of claim 23, wherein said light intensitysensing apparatus comprises a silicon diode sensor which produces alight intensity measurement.
 38. The apparatus of claim 37, furthercomprising a power meter for converting the light intensity measurementinto a digital form.
 39. The apparatus of claim 23, wherein said lightintensity sensing apparatus comprises a photo-multiplier tube withoutput signal means.
 40. The apparatus of claim 23, wherein said primaryhigh energy beam is a sheet beam.
 41. The apparatus of claim 40, whereinsaid sheet beam has a width at least as wide as a width of saidsubstrate.
 42. The apparatus of claim 23, wherein said material is aphoto-resist material.